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HiDtDgraphi 

Sciences 

Corporatio] 


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PhotDgraphic 

Sciences 
Corporation 


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UPON    THK 

PRODUCTION   OF  SOUND   BY   RADIANT 

ENERGY. 


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By  Alexander  Graham  Bkij,, 


[A  Paper  read  before  the  National  Aoadeniyof  Scieuoea,  April,  '21,  1881.  J 


In  a  paper  read  hefore  tlie  Aineriiian  Assoriatioii  fur  t!ie 
Advancement  of  Science,  lust  AugUHt,  I  described  certuiii  ex- 
perimentH  made  l»y  Mr.  Sumner  Tainter  and  myself  which  had 
resulted  in  the  construction  of  a  "  Photophone^''  or  apparatus 
for  the  production  of  sound  by  light;*  and  it  will  he  my  oh- 
je(!t  to-day  to  descrrihe  the  progress  we  have  made  in  the  inves- 
tigation of  photophonic  phenomena  since  the  date  of  this  com- 
munication. 

In  my  Boston  paper  the  disciovery  was  announced  that  thin 
disks  of  very  many  different  substances  emitted  sounds  when 
exposed  to  the  action  of  a  rapidly-interrupted  beam  of  sunlight. 
The  great  variety  of  material  used  in  these  experiments  led  me 
to  believe  that  sonorousness  under  such  circumstatices  would  be 
found  to  be  a  general  property  of  all  matter. 

At  that  time  we  had  failed  to  obtain  audible  effects  from 
masses  of  the  various  substances  which  became  sonorous  in  the 
condition  of  thin  diaphragms,  but  this  failure  was  explained 
uf>on  the  supposition  that  the  molecular  disturbance  produced 

*  FroceediugB  of  Aiuerioau  Awooiatiou  for  the  Advanoement  of  Science, 
Aug.  27th,  1880;  see,  also,  American  Jonniol  cf  Science,  vol.  xx,  p.  3or>; 
Journal  of  the  American  Electrical  Society,  vol.  iii,  p.  3 ;  Journal  of  the 
Society^ of  Telegraph  Engineera  and  Electriciaas,  vol.  iz,  p.  401 ;  Annalea  de 
Ghimie'et  do  Physique,  toL  xxi. 


■J. a 


ji-ii^ 


Ii.v  till'  li^lif  wiiH  I'liirrty  II  Kiirfiico  iicfiun,  iitid  timt  iiikIci'  t\w 
(•in  lllll^lllll(•»'^  i>f  tlic  cxixTinit'iith  tlic  vilinitioii  liml  tu  lie  fniiiK- 
iiiittc.l  tliniiijili  tilt  iiuiKH  <il'  tlif  sul)hfaiifi'  iiMMtk-r  tu  ivrtfct  tho 
far.  It  WHS  tlicrcfurt?  HU|)j».mi'(l  that,  if  wo  (oiild  Iciul  to  tlic  our 
air  tiiat  was  <iircctly  in  coiitiu-t  with  tlu'  illiiiiiiiititcci  Hiirfacc, 
loialcr  suiiikIh  iiii^ht  !>(>  i^htaincd,  and  solid  niassfK  lie  fuiind  to 
he  as  siiMnruus  as  thin  diai)l!rai^nis.  The  first  I'Xju'riiiicnts  niiidc 
til  vcritv  this  hvpothcsis  jMtintt'd  towards  sncci-ss.  A  hciim  of 
sunlight  was  fot-nswi'd  into  (nic  i-nd  of  un  ojmmi  tnltc,  tho  car 
lichi"^  j)lac('d  at  tho  other  <'nd.  Ujmhi  intrrrni>tin;^  the  lu'iun,  a 
dear,  nnisical  tone  was  heard,  the  pitch  of  which  de])eiided  upon 
the  fre([nency  of  the  iiiterriij)tion  of  the  lijiht  an<l  the  hmdnosa 
upon  the  inateriiil  coni])osin^  the  tidie. 

At  this  stuf^c  onr  experiments  were  interrupted,  as  cinMiiu 
stances  called  ine  to  Knro]>e. 

While  in  I'arin  a  ni'w  form  of  th(>  experiment  occurred  to 
my  mind,  which  wotdd  not  only  enable  us  to  investipite  the 
sounds  ])rodueed  hy  manses,  hut  would  also  permit  ns  to  test 
the  njore  general  proposition  that  HonoroiixnexH,  ntnliir  th<'.  in- 
HidiKi.  of  intvrinittrid  lii/ht,  ix  a  projhrtifi'oiiiiiioii  to  (til  inntler. 

The  sid)stance  to  lie  tested  was  to  he  placed  in  the  interitir 
of  a  transparent  vessel,  made  of  some  material  which  (like  ghiss) 
is  transparent  to  light,  hut  practically  oj^ayue  to  sound. 

Under  such  circumstances  the  light  (rould  get  in,  hut  the 
sound  priuluced  hy  the  viltration  of  the  suhstance  <'ould  not  get 
out.  The  audible  effects  coiild  be  studied  by  placing  the  ear 
in  communication  with  the  interior  of  the  vessel  hy  means  of  a 
hearing  tid)e. 

Some  preliminary  experiments  were  nuidc  in  Paris  to  test 
this  idea,  and  the  results  were  so  promising  that  they  were  eom- 
niunicated  to  the  PVench  Academy  on  the  11th  of  October,  1880, 
in  a  note  read  for  me  l>y  M.  Antoine  IJreguet.*  Shortly  after- 
wards I  wrote  to  Mr.  Tainter,  suggesting  that  he  should  carry 
on  the  investigation  in  America,  as  circumstances  prevented 
me  from  doing  so  myself  in  Europe.  As  these  experiments 
seem  to  have  formed  the  common  starting  point  for  a  series  of 
independent  researches  of  the  most  inaportant  character,  car- 
•  Compter  JienduM,  vol.  xcl,  p.  595. 


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8 

rie<l  on  Kiinnltam  oii.^ly,  in  Atiicririi  liy  Mr.  Taintcr,  ii)id  in 
Knro|)f  liy  M.  Mcn-iidicr,*  I'rof.  TyniialKt  W.  K.  Konttr«'n4 
iind  W.  II.  i'ntrc,  .-^  I  iiiny  lie  pcrniitted  to  (|ii(it»'  liniii  my 
letter  to  Mr.  Taintcr  llic  |>a.ssaj;c  dcM-riiiinj;  the  c,\|)erinientH 
referred  to: 

"  MhTkoi'oi.rpAN  UoTKi.,  RuK  Camiion,  Pakih, 

"   Nor.  2,  18S0. 
"  Dkak  Mk.  Taintkk  : 

•  *  *  "I  liuve  devised  a  inetliof!  td'  ]»rodiicinir  suiinds  liy 
"  the  action  of  an  intermittent  lieam  of  lij;ht  from  suhstaiices 
"  that  eaimot  lie  olitaincd  in  the  shape  of  thin  diaphra^'ins  or 
"  in  the  tnlmlar  furni ;  indeed,  the  method  is  specially  adapted 
"  to  tchtin;;  the  jrencralily  td'  the  phenomenon  we  havti  discov- 
"  ered,  as  it  can  lie  adajite  1  to  solids,  li(jiiids,  and  j^ases. 

"  Place  the  sidistaiK-c  to  he  experimented  with  in  a  j^lass  test- 
"  tnlie,  (Miimect  a  rnlihcr  tuli((  with  the  month  of  the  test-tnlie, 
"  jilacinj^  tli('  other  end  of  the  pipe  to  the  ear.  Then  focns 
"  the  intermittent  lieam  npuii  rhe  siilistaiice  in  the  \\\W\  I  have 
"  tried  a  lariic  numlier  of  sidistances  in  this  wav  with  lireat 
"  success,  althoiii^h  it  is  extremely  ditlicnlt  to  i;et  a  jrlimpsc!  of 
"  the  sun  here,  ami  when  it  does  shine  the  intensity  of  the  li;;lit 
"  is  iHit  to  lie  compared  with  that  to  lie  ohtained  in  Wasliin^- 
"  ton.  I  <;ot  splendid  etfccts  fnnn  crystals  of  liichromate  of 
"  potash,  ci'ystals  of  sid])hate  of  coppei',  ami  from  t<iliacco 
"smoke.  A  whole  ci<;ar  jilaced  in  tlie  test-tnlic  ]irodnced  a 
"  very  loud  sonnd.  I  coidd  not  hearanythiny  from  plain  wafer, 
"  lint  when  the  water  was  discolored  with  itd<  a  feelile  sonnd 
"  was  heard.  1  would  suf^j^est  that  you  nnjjht  rejieat  these  ex- 
"  periments  and  extend  the  residts,"  »fee.,  »&e. 

lU'perinicatK  with  SoUila. 

Upon  my  return  to  Wasliin<j;ton  in  the  early  part  of  .,  !'uary,|] 
Mr.  Taintcr  communicated  to  me  th(>  results  of  the  expe  'nuMits 
he  had  made  in  my  lalioratory  during  my  alisence  in  Europe. 

*  "  Notes  ou  liadinphony,"  Cimpten  Rendm,  Die.  (!  and  13,  IHHO;  Feb.  21 
and  28,  1881.     See,  hIho,  Journ<i.l  (le  I'/ii/m'qiif,  vol.  x,  p.  ",;}. 

+  "  Action  of  nn  Intermittent  Heiiin  of  UadiHiit  H<!at  upon  OaseouB  Mailer.'' 
I'roc.  Royal  Siteiety,  Jiiu.  13,  1881,  vol.  xxxi,  p.  307. 

X  "On  the  tones  which  ariKe  from  the  intciriiiittunt  illumination  of  a  giiH." 
8e»!  AnnaUn  der  I'liyn.  und  Vhemic,  Jan.,  IKSi,  No.  1,  p.  l.'i.'). 

§  "  On  tho  Conversion  of  Radiant  Energy  into  Sonoroiis  Vibrations."  Proc. 
Royal  Society,  March  10,  1881,  vol.  xxxi,  p.  .'ilMi. 

II  On  the  7th  of  January. 


Hi'  liiul  ('i»imr.t!ii('C(l  )>.v  oxiiiniiiing  the  sonorous  proportien  of 
ii  vast  iiiiiiil>t'r  of  suhstaiiccfi  oiicloBcd  in  tost-tul»es  in  u  simple 
ciiipiriciil  Hoareh  for  loud  effec^ts.  He  was  tlius  led  {rrachiHlly 
to  the  discovery  that  cotton-wooi,  worsted,  silk,  and  tibrous 
materials  jreiierally,  produced  mu('h  louder  soimds  ti\an  hard 
rijfid  iKKJleslike  crystals,  or  diaphragms  such  as  we  had  hitherto 
used. 

In  order  to  study  the  effects  under  better  circumstances  he 
eu'-losed  his  materials  in  a  conical  cavity  in  a  piet^e  of  lirass 
dosed  Ity  a  Hat  phite  of  ghiss.  A  iirass  tul)e  leading  into  the 
cavity  served  f<»r  connecti(»n  with  the  hearing-tuhe.  When 
this  conical  cavity  was  stuffed  with  worsted  or  other  iibrous 
material-  the  so\mds  produced  were  nuich  louder  than  when 
a  test-ti.oe  was  employed.  This  form  of  receiver  is  shown  in 
Fig\ii'e  I. 

Mr.  Tainter  next  collected  silks  and  worsteds  of  diiferent 
colors,  and  speedily  four.d  that  the  darkest  shades  prodiiced  the 
liest  effects.     Black  worsted  especially  gave  an  extremely  loud 

sound. 

As  white  cotton-wool  had  proved  itself  equal,  if  not  superior, 
to  any  other  white  til)rous  material  before  tried,  he  was  anxious 
to  obtain  (!olored  specumens  for  comparison.  Not  having  any 
at  hand,  however,  he  tried  the  effect  of  darkeiung  some  (;otton- 
wool  with  himp-black.  Such  a  marked  reinforcement  of  the 
sound  residted  that  he  wks  induced  to  try  lamp-black  alone. 

About  a  teaspoonful  of  lamp-ldack  was  placed  mi  a  test-tube 
and  exposed  t<t  an  intermittent  beam  of  sunlight.  The  sound 
prodaced  was  much  louder  than  any  heard  befor 

Upon  smoking  a  piece  of  plate-giass,  and  holding  it  in  tl  e 
intermittent  beam  with  the  lamp-ldack  surface  towards  the  sun, 
the  sound  produced  was  loud  enough  to  be  heard,  with  atten- 
tion, in  any  part  of  the  room.  With  the  lamp-l)lack  surface 
turned  from  the  sun  the;  sound  was  nnurh  feebler. 

Mr.  Tainter  repeated  these  experiments  for  me  immediately 
upon  my  return  to  Washington,  so  that  1  might  verify  his 
results. 

Upon  smoking  the  interior  of  the  conical  cavity  shown  in 
Figure  1,  and  then  exposing  it  to  fh^  intermittent  beam,  with 


l» 


umin 


tm  i>iiti»iw*« 


^ 


us  propcrfie«  of 
l»f'H  ill  u  siiiiple 
«  I«'d  j-'nuhmlly 
!•<,  fuid  fil)roii8 
inlri  than  Jmrd 
ive  hud  liitliertu 

MuuKtiiniieH  lie 
)iece  of  l.rasH 
adiufr  into  tJie 
:-tul»t'.  Wlion 
other  flln-oiLs 
ler  than  wlien 
Gi"  is  shown  in 

8  of  different 

produced  the 

xtrenielj  loud 

not  superior, 
0  WHS  anxious 
't  havin-r  -.my 
'  some  cotton- 
^'luent  of  the 
fick  alone. 
'H  a  test-tube 
Tli<)  sound 

»K  it  in  t}e 
iirds  (he  sun, 
J  with  atten- 
Ijick  surface 

iinmediatelj 
t  verify  his 

^J  shown  in 
heani,  witlj      ^ 


mhMI 


the  {jjlass  lid  in  position  as  shown,  the  effect  was  jjorfectlj' 
startling.  The  soinid  was  so  loud  as  to  be  actually  painful  io 
an  ear  placed  doseiy  against  the  end  of  the  hearing-tube. 

The  sounds,  however,  we.e  sensibly  louder  when  we  j)laced 
some  smoked  wire  gauze  in  the  receiver,  as  illustrate'l  in  the 
drawing,  Figure  1. 

When  the  beam  was  thrown  into  a  resonator,  the  interior  of 
which  had  been  smoked  over  a  lamp,  most  curious  alternations 
of  sound  and  silence  were  observed.  The  interi'uiiting  disk 
was  set  rotating  at  a  high  rate  of  speed,  and  was  then  allowed 
to  come  gradually  to  rest.  An  extremely  feeble  nuisical  tone 
was  at  first  heard,  which  fell  in  pitch  as  the  rate  of  interrup- 
tion grew  less.  The  loudness  of  the  sound  produced  varied  in 
the  most  interesting  manner.  Minor  reinforcements  were  con- 
stantly occurring,  which  became  more  and  more  marked  as  the 
true  pitch  of  the  resonator  was  neared.  When  at  hist  the  fre- 
quency of  interruption  corresponded  to  the  fre(iuen((y  (jf  the 
fundamental  of  the  resonator,  the  sound  was  so  loud  that  it 
might  have  been  heard  by  an  audience  of  lumdreds  of  people. 

The  effects  produced  by  lamp-l)lack  seemed  to  me  to  be  \ery 
extraordinary,  especially  as  1  had  a  distinct  recollection  of  ex- 
periments made  in  the  summer  of  1880  with  smoked  diaphragnis, 
in  which  no  such  reinforcement  was  noticed. 

Upon  examining  the  records  of  our  past  photophonic  experi- 
ments we  found  in  vol.  vii,  p.  57,  the  following  note: 

"  Experiment  V. — Mica  diaphragm  covered  with  lamp-blat^k 
on  side  exposed  to  light. 

"Result :  distinct  tound  about  same  as  without  lampblack. — 
A.  G.  li.,  July  \m,  1880. 

"Verified  the  above,  but  think  it  somewhat  louder  than 
when  used  without  lamp-black." — S.  T.,  July  18th,  1880. 

Upon  repeating  this  old  experiment  we  arrived  at  the  same 
result  as  that  noted.  Little  if  any  augmentation  of  sound  re- 
sulted from  smoking  the  mica.  In  this  expenment  ti»e  effect 
was  observed  by  placing  the  mica  diaphragm  against  the  ear  and 
also  by  listening  through  a  hearing-tube,  one  t'vA  of  whi.'h  was 
closed  by  the  diaphragm.     The  sound  was  found  tt>  be  more 


""'Mil  K 


!    * 


i      I 

I     !i 
■     11 


r> 

aiidililc  through  the  froo  air  when  t)»e  car  was  placed  as  near 
to  the  hini|)  hhirk  surface  as  it  cduld  l»e  Itroiiirht  without 
hIuvUii"?  it.  Tlius  the  vihratious  produced  in  hunp-hhic'v  under 
tile  aliove  circumstances  do  not  appear  tti  he  comnuinicated  to 
any  very  appreciahle  extent  to  the  diaphra<;ni  on  which  the 
lanip-ldat^ii  is  deposited. 

At  tlie  time  of  my  comnnmi<  ation  to  the  American  Associa- 
tion I  had  lieen  unaitki  to  satisfy  myself  that  the  substances 
which  had  'oecome  sonorous  under  the  direct  influence  of  inter- 
inittent  sunlij^ht  \v(4'e  caj>altle  <tf  reproduciniij  the  sounds  of 
articidate  speech  under  tlie  action  of  an  undulatory  heam  from 
our  i)hotophonic  transmitter.  The  difliculty  in  ascertaining 
this  will  he  \niderstood  i)y  considering  that  the  sounds  emitted 
hy  thin  diapiiragms  and  tuhes  were  so  feeble  that  it  was  im- 
prac,tical)le  to  produce  audible  cirects  from  substances  in  these 
conditions  at  any  considerable  distance  from  the  transmitter; 
but  it  was  e(jually  injpctssible  to  judge  of  the  effecrts  produced 
by  our  artimlate  ti'ansmittei*  at  a  short  distance  away  because 
the  speaker's  voice  was  directly  audible  thntugh  the  air.  The 
extremely  loud  sounds  pnxluced  from  lamp-black  have  enabled 
us  to  demonstrate  the  feasibility  of  using  this  suiistance  in  an 
articulating  photophone  in  phice  of  the  electrical  receiver  for- 
merly employotl. 

The  drawing  (Fig.  2)  illustrates  the  mode  in  which  the  experi- 
ment v,'as  conducted.  The  diaphragm  of  the  transmitter  (A) 
was  only  5  centimetres  in  diameter,  the  diameter  of  the  re- 
ceiver (B)  was  alao  5  centimetres,  and  the  distance  between  the 
two  was  40  metres,  or  800  times  the  diameter  of  the  transmit- 
ting diaphrtigm.  We  were  unable  to  experiment  at  greater 
distances  without  a  heliostat  on  account  of  the  ditiiculty  of 
keeping  the  light  steadily  directed  on  the  receiver.  Words 
and  sentences  spoken  intt)  the  transmitt"r  in  a  low  tone  of  voice 
were  audibly  reproduced  by  the  lamp-black  receiver. 

In  Fig.  8  is  shown  a  mode  of  interrupting  a  beam  of  sunlight 
for  producing  distant  etfet-ts  without  the  use  of  lenses.  Two 
similarly-perforated  disks  are  employed,  one  of  which  is  set  in 
rapid  rotation,  while  the  other  remains  stationary.  This  form 
of  interrupter  is  also  admirably  atlapted  for  work  with  at  tiflcial 


JSkm 


iicofl  as  near 
d>t  without 
-1>1}ica  uiidfr 
rn.nicated  to 
1   which  tlio 

'an  Associa- 
'  wiil).stan(!e8 
we  of  iiiter- 
i  sounds  of 

heani  from 
iscertairiinij 
lids  emitted 

it  was  iin- 
^'08  in  these 
ransiniiter; 
9  prodneed 
ay  because 
!  air.  T)ie 
ve  enabled 
ance  in  an 
eeiver  for- 

the  experi- 
tnitter  (A) 
af  the  re- 
tween  the 
transmit- 
■t  greater 
Hculty  of 
Words 
3  of  voice 

■  sunlight 
Js.  Two 
I  is  set  in 
'his  foi-in 
a'tificial 


■■■i* 


MM 


■m 


11 


lif^ht.  The  receiver  illuKtriited  in  the  druwirif^  ooiisiHtH  of  a 
piirulxilic!  reflector,  in  the  focnis  of  which  i^  i)hu^e(i  a  j?hi8H  ves- 
Kci  (A)  contiiinin^  lainj>-l)hick  or  other  sensitive  suitstance,  and 
connected  with  a  l.earing-tuhe.  The  heain  of  light  is  inter- 
1  ii])ted  l»y  its  passage  tln'oiigli  the  tw(»  sh>tted  disks  shown  at  H, 
and  in  operating  the  instnnnent  nuisical  sigc.als  like  the  <lots 
and  ilashes  of  the  Morse  ali)hal)et  arc  produced  from  the  scMisi- 
tive  receiver  (A)  l>y  sliglit  motions  of  the  mirror  (C)  a1»out  its 
axis  (I\) 

In  place  of  the  parabolic  refle(^tor  shown  in  the  tigure  a  ironi- 
cal reflector  like  that  recouunended  !)y  Prof.  Sylvanus  Thomp- 
son* can  he  used,  in  which  case  acylintlrical  glass  vessel  would 
he  preferaltle  to  the  flask  (A)  shown  in  tlu^  flg\n'e. 

In  regard  to  the  sensitive  materials  that  can  be  employed, 
our  experiments  indicate  that  in  the  case  of  solids  the  physical 
condition  and  the  color  markedly  influence  the  intensity  of 
the  sonorous  effects.  Tlte  loadesl  mionh  are  pyodiicet/  from 
snhxtances  in  a  loose,  porous,  sjHuxjy  condition,  and  from  those 
that  have  the  darkest  or  most  ahsorhent  colors. 

The  materials  from  which  the  host  effects  have  been  obtained 
are  cotton-wool,  worsted,  fibrous  materials  gene^'ally,  cork, 
sponge,  platinum  and  other  metals  in  a  spongy  condition,  and 
lamp-black. 

The  loud  sounds  produced  from  such  substances  may  per- 
haps be  explained  in  the  following  matnier:  Let  us  consider, 
for  example,  the  case  of  lamp-black — a  substance  which  be- 
comes heated  by  exposure  to  rays  of  all  refrangibility.  I  look 
upon  a  mass  of  this  sulistance  as  a  sort  of  sponge,  v.ith  its  pores 
filled  with  air  instead  of  water.  When  a  beam  of  sunlight 
falls  upon  this  mass,  the  particles  of  lamp  black  are  heated,  and 
consequently  expand,  causing  a  contraction  of  the  air-spaces 
or  pores  among  them. 

Under  these  circumstances  a  pulse  of  air  shoild  be  expelled, 
just  as  we  would  squeeze  out  water  from  a  sjxinge. 

The  force  with  which  the  air  is  expelled  nuist  be  greatly  in- 

'  (!reased  by  the  expansion  of  the  air  itself,  due  to  contact  with 

the  heated  particles  of  lamp-black.     When  the  light  is  cut  off 

♦Phil.  Mag.,  April,  1881,  vol.  xi,  p.  28(). 


ffi^^^p^igjaig^^i!i«5jalia»a^^^ieai 


t; 


12 


•li- 


the converse  procews  tiikcR  pliico.  The  liunp-l)la«:k  particloH 
cool  iintl  contract,  tlnis  enlarging;  the  air  Kpaces  among  them, 
an<l  tlie  enehjwed  air  alno  hecoinett  co(tl.  Uncier  these  circum- 
stancert  a  partial  vacuum  shouM  lie  formed  among  the  particles, 
and  the  (jiit-jde  air  would  tiien  he  altsorhed,  as  water  is  hy  a 
spougci  when  the  pressure  of  the  hand  is  rem«)ved. 

I  imagine  that  in  some  such  manner  its  this  u  wave  of  con- 
densatiim  is  started  in  the  atmosphere  eacli  time  a  beam  of  sun- 
light falls  upon  lamp-hhu^k,  and  a  wave  of  rarefaction  is  origi- 
nated when  *he  light  is  cut  off.  We  ain  thus  uiniei'dani/ 
liow  it  iif  that  a  suhntdiice  like  liiiiip-hlnek  pi'txhirett  intense 
sonoruits  aibrationn  in  the  iturroiindinij  di/',  while  tit  the  sn/ne 
tim£  it  emnimtnieates  a  very  feehle  vihrtition  to  the  iliivphrngm 
or  Kolid  bed  upon  which  it  rests. 

This  (uirious  fact  was  independently  observed  in  England  ity 
Mr.  Preecie,  and  it  led  him  to  ([uestion  whether,  in  our  experi- 
ments with  thin  diaphragms,  the  sound  heard  was  due  to  the 
viliratioii  of  theilisk  or  (as  Prof.  Hughes  had  suf^gssted)  to  the 
expansion  and  contraction  of  the  air  in  contuct  «•  ith  the  disk 
confined  in  the  cavity  behind  the  diaphragm.  In  his  paper  read 
before  the  Royal  Society  on  the  lOth  of  March,  Mr.  Preece  de- 
scriltes  experiments  from  which  he  claims  to  have  proved  that 
the  effects  are  wholly  due  to  the  vibrations  of  the  contined  air, 
and  that  the  disks  Uoxjmi  'iuhrtite  ui  nil. 

I  shall  briefly  state  my  reasons  for  disagreeing  with  him  in 
this  conclusion : 


\ 


^ 


1.  When  an  intermittent  beam  of  sunlight  is  focnssed  upon 
a  sheet  of  hard  rubber  or  other  material,  a  nuisical  tone  can  l»e 
heard,  not  (mly  by  placing  the  ear  inmiediately  behind  the  part 
receiving  the  lieara,  hut  by  placing  it  against  any  portion  of  the 
sheet,  even  though  this  may  be  a  foot  or  more  from  the  place 
acted  iipon  by  the  light. 

2.  When  the  beam  is  thrown  upon  the  diaphragm  of  a  "  Blake 
Transmitter,"  a  loud  musical  tone  is  produced  by  a  telephone 
connected  in  the  same  galvanic  circuit  with  the  carbon  button, 
(A,)  Fig.  4.  Good  effects  are  also  produced  when  the  carlxm 
button  (A)  forms,  with  the  battery,  (B,)  a  portion  of  the  pri- 
mary circuit  of  an  induction  c<»il,  the  telephone  (C)  being 
placed  in  the  secondary  circuit. 


lilJUUimiu:- 


^ 


parti«-lofl 
Boiifv  them, 
rte  <^ir<Muii- 
piirticli's, 
'1"  IH    I)y  a 

e  of  (fon- 

nil  of  81111- 

i«  origi- 

>i<ft;t'Kt.,i)i(/ 

i'f<  intense 

the  unine 

<iphi'iHjm 

i;1hm(1  |»y 
ir  oxpeii- 
iie  to  tlie 
5d)  to  the 

tlie  disk 
ipei-  read 
ceeee  de- 
ved  tlmt 
ined  air. 


I  liiin 


m 


id  upon 
can  he 
he  part 
1  of  tlie 
e  pla(!e 

'Bhike 

sphone 

mtton, 

^arhon 

e  pri- 

being 


i 


mmmmmmmm 


I  I 


!<! 


¥ 


riiilHii 


f-i 


tammmmmmmmmmmmmtm 


^ 


M 

In   tlu'M'   CIIM'S   tin-  WniMlfll    ll(»\   tllltl    ll|IMltll-|)i<'<-0  of  f|u«   trilllH- 

iiiittor  hlioiilil  l)f  rniKivcd,  Hotliii'  im  iiir ciiviticH  may  l>«>  left  on 
citln-r  hific  nf  tlic  iliii|ihriini. 

//  In  t  rii/rtif,  fln'i'i'fiiri',  tlint  in  tlir  mm  of  thtii  tl'isltH  n  rml 
r'lhi'tiflnii  of  till'  tlliifthi'tiifiii  is  ini/ti'i/  /ii/  thr  iirtion  of  tin'  iii- 
ti'riiiifti  lit  hi'iiin,  iiiilijMiiili  nth/  of  mti/  <:i-/niiii<ioii  uml  rontnir- 
tloii  of  thv  air  roit^ft'iifi/  in  thi'  rui'ifif  hiliiinl  tin'  ilin/thrtn/ni. 

Lonl  Iliiylci^h  Iihh  hIimwm  iniitlitMniitic'iilly  tliiit  u  t<>-iin<i-fi'<> 
\  iliration,  nf  hutticicnt  tiniplifiiilr  to  |>r<Mluco  im  liiMlililc  Hoiirxl, 
would  rcsnit  from  a  |>(>i'ioi|iral  commuMication  and  altntraction 
of  heat,  and  lie  ways:  "We  may  coiiclnilf,  I  lldiik,  tliat  tlicrc 
"  Ih  at  jtrrHcnt  no  rcahon  for  <lis4-ardiii^  flKi  ohvionHoxplanation 
"  tlmf  thr  hoiiikIh  in  (piCHtion  iiro  due  to  tlic  Ix-ndiiij;  of  \\\v 
"  phifos  inidpr  imci|nal  licatin^."  (Nature,  xxiii,  j».  274.) 
Mr.  I'rPfct',  however,  seeks  to  prove  tliat  the  .sonorous  effects 
(;iUniot  l)(^  exphtiued  upon  tliis  su])position  ;  hut  his  experimental 
datii  are  not  sutticient  to  support  his  eonelusion.  Mr.  I'reeei^ 
fxpeeted  that  if  Lord  Ilaylei«i;irs  explanation  whs  eorrecrt,  the 
expansion  and  eontraetion  of  a  tliiii  stri]>  under  the  intluenee 
of  an  intermittent  lieam  could  Ite  caused  to  open  and  closu  a 
^Hlvani<^  circuit  ho  as  to  prcMluce  a  musical  tone  from  u  tele- 
phcme  in  tiie  circuit.  Hut  this  wuh  an  inade(pnite  way  to  test 
tlie  point  at  issue,  for  Lord  Ilavleijjli  has  sh(»wn  (Proc.  of  Roy. 
So(\,  1n77)  that  an  aiidiMe  sound  t^m  l»e  produced  liy  a  vilira- 
tion  whose  amplitude  is  /r.v.v  t/mn  a  trn-niilliont/i  of  n  ccntinu;- 
trn,  and  eortaJidy  such  a  vil»rati(Hi  as  that  w(Mild  not  have  suf- 
ficed to  operate  a  "  nuike-and-hreak  (rontai^t"  like  that  used  hy 
Mr.  I'reece.  The  nej^ative  results  ohtained  l>y  him  (!atmot, 
therefore,  l>e  considered  (uaiclusive. 

The  following  experiments  (devised  hy  Mr.  Tainter)  have 
s;iven  results  decidedly  more  favorahlo  to  the  theory  of  Lord 
Riiyk'if^h  than  to  that  of  Mr.  Preece : 

1.  A  strip  (A)  similar  to  that  used  in  Mr.  Prcece's  experi- 
ment was  attached  firmly  to  the  centre  of  an  iron  <liaplira>^m, 
(li,)  as  shown  in  Fij^ure  5,  and  was  then  pulled  taut  at  rii^ht 
angles  to  tlie  plane  of  the  diaphragm.  When  the  intermittent 
heam  was  focussed  npon  the  strip  (A)  a  clear  musical  tone 
could  1)0  heard  l)y  applying  the  ear  to  the  heariug-tuhe  (C.) 


uuu 


fi 
'I 


if  ■, 


;i: 


18 

77i!,s  K''<iiii</  til  iixlii'Kfc  it  I'lipUl  I'.rjxinKiiin  miil  Ciniti'iicfiini  of 
till  siihxtiiiiiw  iniil''r  ti'iiil . 

l>iit  a  vilmitioii  of  \\\v  (liaplira^fin  (H)  would  also  have  re- 
sulted if  the  thin  strip  (A)  had  ae(jiiirc<l  a  to-aiid-fro  motion, 
(ine  eitliiT  to  the  direct  impact  of  the  Iteam  or  to  the  sudden 
exjiansiou  of  the  air  in  contact  with  the  strip. 

'J.  To  test  whether  this  had  lieen  the  casr'an  additional  strip 
(1))  was  attached  Ity  its  central  point  oidy  to  the  ftrip  under 
trial,  and  was  then  suhniittcd  to  the  action  of  the  heani,  as 
shown  in  Fif;.  0. 

It  was  presuiiKMi  that  if  the  vi'tration  of  the  dia|)liraj;ni  (IJ) 
had  heen  due  to  a  pnxlinHj  funv  actiufj;  on  the  strip  (A,) 
that  the  addition  of  the  strip  (D)  would  not  interfere  with 
the  effect.  Jiut  if,  on  the  other  hand,  it  had  l>eeu  due  to  the 
loni;iladiiial  expansion  and  contraction  of  the  strip  (A,)  the 
sound  would  cease,  or  at  least  he  reduced.  The  heam  of  light 
fallini:  upon  strip  (D)  was  now  interrupted  as  l)efore  hy  the 
rapid  rotation  of  a  perforated  disk,  which  was  allowed  ti»  come 
frntdually  to  rest. 

No  sound  was  heard  except in<^  at  a  certain  speed  of  rotation, 
when  a  feehle  miisicai  tone  hecanie  audihle. 

This  result  is  conHrmatory  of  the  iirst. 

The  audi!)ility  of  the  effect  at  a  parti-.tular  rate  of  interruption 
siiiif^ests  the  exp'uiiatiiUJ  that  the  strip  I)  had  a  normal  rate 
of  viiiration  of  its  own. 

When  the  frecjnency  of  the  interruption  of  the  lij^ht  corres- 
|toiided  to  this,  the  strip  was  prohaldy  throwji  into  vibration 
after  the  maimer  of  a  tuning-fork,  in  which  case  a  to-aiid-fro 
viiiration  would  i)e  propagated  down  its  stem  or  eentral  support 
to  tlu'  stri]>  (A.) 

This  indirectly  proves  the  value  of  the  experiment. 

The  list  of  solid  suhstances  that  have  I)cen  sui)mitted  to 
experiment  in  my  lahoratory  is  t(»o  long  to  he  (pioted  here,  and 
I  shall  merely  say  that  we  have  not  yet  found  one  solid  hody 
that  has  failed  to  become  sonorous  under  proper  conditions  of 
experiment.* 

Kxperhiunts  init/i   Llijii'ids. 
The  sounds  produced  by  lifpiids  are  much  more  ditlicidt   to 

*  (>Hrli(Mi  and  Uiiii  iiiiuroNcope  glasH  are  lutiutioiiud  in  my  HoNtoii  papur  ati 
ii")ii-resjK)UHiv«'.  and  po  vdored  chlorate  of  potaah  iti  the  coniiiiuniuatioii  to  the 
Fretu'li  Academy,  (CompteK  ItenduK,  vol.  xcl.  p.  .lilS.)  All  theso  HiiliNtaiicbH 
Uavo  Hiiiuti  yielded  tiouDdH  under  uiure  carulul  couditiouH  of  experiment. 


iH 


<'<»i(i'it,tiint  of 

iilsi)  havi'  ri- 
-tVo  iiKtfioii^ 

•    fill'    8II(I(1»-J1 

lifioiial  Kfrip 
■  fi'ip  iiiidfr 
fJu'  hffini,  as 

•I'lira^nii  (U) 
f  f^trip  (A,) 
tl'l-flTC    with 

"  'Ino  t<»  tJic 
'•'P  (A,)  the 
I'iiiii  of  litpjif 
of  ore  |,y  the 

WCi\  to  COIIIO 

of  rotation, 


iiitfrniption 
'oriiial   rate 

i>ht  correH- 
<>  vihratiori 
I  t«>-;iii(l-fi-o 
ti'ul  support 

t. 

I'lnitted  to 
'  Ifi'i-e,  and 

M>li(]    |„„iy 

"litioDH  of 


ffifiilt   to 

"1  paper  hh 
itioii  to  the 
Hii'mtaiices 
tueut. 


19 

observe  than  thorte  produced  l»y  Holids.  The  hij^h  altworptive 
power  poHsessed  hy  most  li(]uidH  would  lead  one  to  expect  in- 
tense  viltrations  from  the  action  of  intermittent  light,  hut  the 
nnniher  of  sonorous  liquids  that  have  so  far  been  found  is  ex- 
tremely limited,  and  the  soun<ls  produced  are  so  feelde  as  to 
be  heard  only  by  the  greatest  attention  and  under  the  i»est  cir- 
cumstances of  experiment.  In  the  experiments  made  in  my 
laboratory  a  very  long  test-tube  was  filled  with  the  liipiid  under 
examination,  and  a  flexible  rubber-tube  M-as  slipped  over  the 
mouth  far  enough  down  to  prevent  the  possibility  of  any  light 
reachijig  the  vapor  above  the  surface.  Precautions  were  also 
taken  to  prevent  rertectictn  from  the  bottom  of  the  test-tube. 
An  intermittent  beam  of  sunlight  was  then  focussed  upon  the 
li<juid  in  the  nud<lle  portion  of  the  test-tube  by  means  of  a  lens 
of  large  diameter. 

licsiUts. 
Clear  water No  so'ind  audible- 
Water  discolored  by  ink Feeble  sound. 

Mercury No  sound  heard. 

Sidphuric  ether* Feeble,  but  distinct  sound. 

Ammonia "         "         "  " 

Animonio-sulphate  of  copper "         "         "  " 

Writing  ink "         "         "  " 

Indigo  in  sulphuric  acid "         "         "  " 

Chloride  of  copper* "         "         "  " 

The  liquids  distinguished  by  an  asterisk  gave  the  best 
sounds. 

Acoustic  vibrations  are  always  much  enfeebled  in  passing 
from  liijuids  to  gases,  and  it  is  prol)able  that  a  form  of  experi- 
ment may  l»e  devised  which  will  yield  better  results  l)y  com- 
municating the  vil)rations  of  the  Ii(piid  to  the  ear  through  the 
medium  of  a  solid  rod. 

Ki'pcri  lite  ids  with  Gaseous  Matter. 

On  the  29th  of  November,  1880,  I  had  the  pleasin-e  of 
showing  to  Prof.  Tyndall  in  ihe  laboratory  of  the  Royal  Insti- 
tution the  experiments  described  in  the  letter  to  Mr.  Tainter 
from  which  I  have  quoted  above,  and  Prof.  Tyndall  at  once 


20 


(ixprt'Hsed  the  opinion  lliat  the  Konnrls  were  due  to  rupid 
('liiin<jes  of  teinj)er!ituro  in  the  Imdy  submittod  to  the  at-tion  of 
the  heuin.  Findin<j  that  no  experiments  Imd  been  made  at  that 
time  to  test  tlie  sonoi-ouK  propertieR  of  different  pisen,  he  sng- 
•jested  filling;  one  te8t-tul>e  with  the  vapor  of  Kidphiiric  ether, 
(a  ijood  alisorltent  of  heat,)  and  another  with  the  vapor  of  bi- 
sidphide  of  carbon,  (a  poor  al)fiorbent,)  and  he  predicted  thai  if 
any  sf)un(i  wan  heard  it  would  be  louder  in  the  former  case  than 
in  the  latter. 

The  experiment  was  innnediately  made,  and  the  result  veri- 
fied the  prediction. 

Since  the  publication  <»f  the  memoirs  of  Rontjijen*  and  Tyn- 
dall  t  we  have  repeated  these  experiments,  and  have  extended 
the  in(|uiry  to  a  miniber  of  other  gaseous  bodies,  obtaining  in 
every  case  similar  results  to  those  noted  in  the  memoirs  re- 
ferred to. 

The  vapors  of  the  following  substances  were  found  to  be 
highly  sonorous  in  the  intermittent  beam:  Water  vapor,  coal 
gas,  Hulphurii!  ether,  alcohol,  ammonia,  amylene,  ethyl  bromide, 
diethylamene,  mercury,  iodine,  and  peroxide  of  nitrogen.  The 
loudest  sounds  were  obtained    from   iodine  and  peroxide  of 


introgcn. 


I  have  now  shown  that  sounds  are  produced  by  the  direct 
action  of  intermittent  sunlight  fr(»m  substances  in  every  physi- 
cal condition,  (solid,  licpiid,  and  gaseous.)  and  the  probability 
is  therefore  very  greatly  increased  that  sonorousness  under  sucrli 
circumstaiH^es  will  be  found  to  be  a  universal  property  of 
matter. 

[Iptin  Sixhatitiitcs  for  Seleniaiu  in  Kleetnenl  Receivers. 

At  the  time  of  my  connnunication  to  the  American  Associa- 
tion the  kuidest  effects  obtained  were  produced  by  the  use  of 
st'leuium,  arranged  in  a  cell  of  suitable  construction,  and  placed 
in  a  galvanic  circuit  with  a  telephone.  Upon  allowing  an  in- 
termittent beam  of  sunlight  to  fall  upon  the  selenium  a  musical 

•  Ann.  der  Phys.  iind  Chem.,  1881,  No.  1,  p.  lf>5. 
t  I'roc.  Roy.  Sec,  vol.  xxxi,  p.  307. 


1  !! 


1 
iJL 


(. 


e  to  rfipid 
)e  action  of 
lade  iit  that 

SeH,  liC  8Ug- 

iiiric  ether, 
upor  of  l)i- 
ctt'd  thai  if 
r  I'Rue  than 

result  veri- 

*  and  Tyn- 
3  extended 
itaining  in 
lemoii's  re- 

)nnd  to  be 
'apor,  coal 
1  hroinide, 
,'en.  The 
jroxide  of 

the  direct 
ery  physi- 
rol  (ability 
nder  hucIi 
:)perty  of 


I  vet's. 

i  Associa- 
lie  use  of 
iid  placed 
ig  an  in- 
i  iniisical 


■  mill  ,  ^      -■■■ 


hi 


fS 


fin 


I 


M*«^^k^i*a 


I 


Pisr-  7. 


/^a 


23 


tone  of  great  intensity  was  produeod  from  tlie  telephone  con- 
nected with  it. 

But  the  weleniiun  was  very  inconHtant  in  its  action.  Two 
pie<'ert  of  seleniinn  (even  of  the  same  Bti<^l<)  seldom  yielded  the 
sami'  resnlts  under  identical  circunistaiK^es  of  annealing,  ikv. 
While  in  Europe  last  autmnn,  Dr.  Chi(^hester  Bell,  of  Univer- 
sity College,  London,  suggested  to  nie  that  this  inc<instancy  of 
result  inight  l»e  due  to  chemii^al  impurities  in  the  selenium  used. 
Dr.  Beli  has  since  visited  uiy  laboratory  in  Washington,  antl 
has  made  a  chemical  examination  of  the  various  samples  of  se- 
lenium I  had  colleirted  from  different  parts  of  the  world.  As 
I  understand  it  to  he  his  intention  to  publish  the  results  of  this 
analysis  very  soon,  I  shall  make  no  further  mention  of  his  in- 
vestigation than  to  state  that  he  has  foun<l  sulphur,  ir«in,  lead, 
and  arsenic  in  the  so-(falled  "selenium,"  with  tra<tes  of  organiir 
matter  ;  that  a  <piantitative  examination  has  revealed  the  fact 
tiiat  sulphur  constitutes  nearly  one  per  cent,  of  the  whole  mass; 
and  thai  when  these  impurities  are  eliminated  the  selenium  ap- 
pears to  be  more  (constant  in  its  action  and  more  sensitive  to 
light. 

Prof.  W.  G.  Adams*  has  shown  that  tellurium,  like  selenium, 
has  its  electrical  resistance  aifected  by  light,  and  we  have  at- 
tempted to  utilize  this  snbstantic  in  place  of  selenium.  The 
arrangement  of  cell  (shown  in  Fig.  7)  was  constru(;ted  for  this 
purpose  in  the  early  part  of  1880;  but  we  failed  at  that  time 
to  obtain  any  indications  jf  sensitiveness  with  a  reflecting  gal- 
vanometer. We  have  since  found,  however,  that  when  this 
tellurium  spiral  is  connected  ii  circuit  with  a  galvanic;  battery 
and  telephone,  and  exposed  to  tlie  "''.tici  of  an  intermittent 
beam  of  sinilight,  a  distim^t  nuisical  tone  is  pi-oduced  by  the  tele- 
phone. The  audible  effect  is  much  increased  by  placing  the 
tellnriuiti  cell  with  the  battery  in  the  primary  (drcuit  of  an  in- 
duction coil,  and  placing  the  telephone  in  the  secondai-y  circuit. 

The  enormously  high  resistance  of  selenium  and  the  ex- 
tremely low  resistance  of  tellurium  suggested  the  thought  that 
an  allo^  of  these  two  substanees  might  possess  intei-mediate 
electrical  properties.  We  have  accordingly  mixed  together 
♦Proo.  Roy.  Soc,  vol.  xxiv,  p.  163. 


24 


11  a. 


Hcloiiiiiin  iui«l  tcUuritun  in  different  jiroportidnH,  and  while  we 
do  ni»t  feel  warninted  ut  the  pi-e.-fent  time  in  niiikiiiij  detlnite 
8tiiteiuent8  eonceriiing  the  resnltw,  I  nmy  say  that  nywh  rilloys 
have  proved  to  he  sensitive  to  the  aetion  of  light. 

It  (KHMirred  to  Mr.  Tainter  hefore  my  return  to  Washington 
last  Jamiary  that  the  very  great  moleciUar  distiirhance  ))ro- 
diiced  in  lamj)-l)la(!k  hy  the  acttion  of  intermittent  siiidight 
should  prodnee  a  corresponding  distnrhaniie  in  an  electrical  cur- 
rent pastied  through  it,  in  which  ease  lamp-Mack  could  he  em- 
ployed in  place  of  selenium  in  an  electrical  receiver.  This  has 
turneil  out  to  he  the  case,  and  the  importan(!e  of  the  discrovery 
is  very  great,  especially  when  we  consider  the  expense  of  su<^l' 
rare  suhstances  as  selenium  and  tellurium. 

The  form  of  lamphlack  cell  we  have  found  most  effec^tive is 
shown  in  F^ig.  8.  Silver  is  deposited  upon  a  plate  of  glass, 
and  a  zigzag  line  is  then  scratched  through  the  tilm,  as  shown, 
dividing  the  silver  surface  into  two  portions  insulated  from  one 
another,  having  the  form  of  two  comhs  with  interhw^king  teeth. 

Ea(!h  comh  is  attached  to  a  screw-eup,  so  that  the  cell  (^an  be 
phuted  in  an  electrical  circuit  when  required.  The  surface  is 
then  smoked  until  a  giKHl  tilm  of  lamp-black  is  obtained,  filling 
the  interstices  between  the  teeth  of  the  silver  combs.  When 
the  lamp-black  cell  is  connected  with  a  telephone  and  galvanic 
battery,  and  exposed  to  the  influen<^e  of  an  intermittent  beam 
of  sunlight,  a  loud  musicfal  tone  is  produced  by  the  telephone. 
This  result  seems  to  be  «liie  rather  to  t!»e  physical  condition 
I,han  to  the  nature  of  the  coiuUuiting  material  employed,  as 
metals  in  a  spongy  condition  produce  similar  effects.  For  in- 
stance, when  an  electrical  current  is  passed  through  spongy 
platinum  while  it  is  exposed  to  intermittent  sunlight,  a  dis- 
tinct musical  tone  is  produced  by  a  telephone  in  the  anme  (cir- 
cuit. In  all  such  cases  the  effect  is  increased  by  the  use  of  an 
induction  coil;  and  the  sensitive  cells  can  be  employed  for  the 
reproduction  of  articulate  speech  as  well  as  for  the  production 
of  musical  sounds. 

W^e  have  also  found  that  loud  sounds  are  produced  from 
lamp-black  by  passing  through  it   an  intermittent  electrical 


\  while  we 
n^  (Ipfinite 
such  uHoys 

V^UHhiri^foii 
ItuiU'P  })ro- 
it  siinliijht 
fti'ical  (Mir- 
II  1(1  he  em 
This  haw 
!  (li8(!overy 
ise  of  sikO' 

effective  h 
!  of  glass, 
as  slu>wn, 
1  froin  one 
:iiig  teeth, 
ell  van  be 
surfacie  is 
led,  fillinijf 
i.     When 
I  galvanic 
;ent  beam 
elephone. 
condition 
»loyed,  as 
For  in- 
i  spongy 
ht,  a  dis- 
same  cii-- 
ise  of  an 
d  for  the 
odiiction 

ed  from 
;lectrical 


f.-; 


8T 

curroi.t ;  and  that  it  ran  l.o  uhimI  an  a  tolcplionic  rccpiver  for 
the  reprodiicticMi  ..f  arti.-uhitc  spwch  by  ('Iwtrifal  iikmuis. 

A  (M.nvonicnt  iikkIc  ..f  aiTaii«,'iii;,'  a  lainp-l.lack  cfll  for  ox- 
poriuuMitul  purpoKCK  is  sliowu  in  Fi^.  5>.  When  an  inti'rnntt.'nt 
current  in  passed  throuj,'h  the  lamp-Mark,  (A,)  or  when  an  in- 
termittent iKiani  of  Hiudit{ht  falls  upon  it  throuijh  the  fjlass 
pkte  B,  a  loud  mtisieal  tone  (ran  he  heard  hy  applying,'  the  ear 
to  the  hearing-tuhe  C.  When  the  li.^'ht  and  the  electrical  .-nr- 
rentact  sinndtaneously,  two  nnisical  tones  are  perceived,  \vlii<-h 
produce  heats  when  nearly  of  the  same  pitch.  By  prt.per  ar- 
rangements a  c.inplete  interference  of  sound  can  undoubtedly 
be  produced. 

Upon  the  Meimvmttent  of  the  Simoroiis   h'ferts    l^in^hmil  l»J 
J)lf event  Snliddiieex. 

We  have  observed  that  different  substances  produce  sounds 
„f  very  different  intensities  under  similar  (iircumstances  of  ex- 
iMM-iment,  and  it  has  appeared  to  us  that  very  vahudde  informa- 
tion n.ight  be  obtained  if  we  could  measure  the  audible  effects 
produce'il.  For  this  purpose  we  have  (-..nstructed  several  differ- 
ent forms  of  apparatus  for  studying  the  effects,  but  as  our  re- 
searc-hes  ar'.  not  yet  complete,  I  shall  <-onHne  myself  to  a  sun- 
pie  description  of  m.me  of  the  forms  of  apparatus  we  have  tie- 
vised.  „ 

When  a  beam  of  light  is  brought  to  a  focus  by  means  ot  a 
lens,  the  beam  divergi.ig  from  the  focal  point  becomes  weaker 
as  the  distance  increases  in  a  (calculable  degree.  Hence,  if 
we  can  determine  the  distances  from  the  focal  point  at  which 
two  different  substances  emit  sounds  of  etjual  intensity,  we  can 
calculate  their  relative  sonorous  powers. 

Preliminary  experiments  were  made  by  Mr.  Tainter  during 
my  absence  in  Europe  to  a8(certain  the  distance  from  the  focal 
point  of  a  lens  at  which  the  sound  produced  l)y  a  substance 
became  inaudible.  A  few  of  the  results  obtained  will  show 
the  enormous  differences  existing  between  different  substances 
in  this  respect. 


28 

Dititnnve. fi'iHit  tWnl  l*i>'ni(  i[t'  hnn  at   irli'nh    Simiiilx   Ixcnini' 
liudidlhle  ii'liJi  Dijf'irtnt  SuhHtunnx. 

Zinc  <iiii|)liriii;iii,  (polished) l.r>l  in 

Iliinl  nililxT  (liiiplini;;!!! !.!♦(»  " 

Till  foil                   "           2.00  " 

T»'l('|)li<iiic             "          (.lu|»iiiiu>(l  iron) 2.15  " 

Zinc                         "          (inipolishcd) 2.15  " 

White  silk,               (in  rccciviT  hIiowii  in  Ki^.  1.).  .  .  .  ;5.1()  " 

Whito  woiKtHl,             "                   "       »        u      4()i  u 

Yellow  worsted,             "                    4»        u         u      4  o»',  " 

VeUow  Hilk,                   "                   "       "        "      +.l;{  " 

White  cottoii-wool,       "                   »       u        u      4  38  u 

(ireeii  silk,                       "                     u        u         u      4  52  u 

liliie  worsted,                  "                     »i        u         u      4  ,5,)  ,, 

Tin-ple  silk,                    "                    .*        ^^         a      4^^^  " 

l?rown  silk,                    "                   i*       '»        "...  5.<>2  " 

lilitck  silk,                      "                   "       "        "...  5.21  " 

Red  silk,                         "                    "        "         "      .  .  .  .  5.24  " 

IViaek  worsted,               "                    "        "         "      ('..50  " 

Jjiunp-Maek.      In  this  (rase  the  limit  of  aiidiliilitv  <'oiild 

Mot   he  determined  on   aeconnt  of   want  of   space. 

Sound  perfectly  an-lihle  at  a  distance  of 10.00  " 

Mr.  Taint<>r  was  convinced  from  these  e.xju'riments  that  this 
tiehl  of  resoach  promised  valiiahle  results,  and  he  at  once  de- 
vised an  apparatus  for  stuflyiii;^;  the  elYccts,  which  he  de.scrihed 
to  me  upon  my  return  from  Eurojx".  Th;  uppuiatus  has  since 
heen  constructed  and  i  take  great  pleasure  in  showing  it  to  you 
ttt-day. 

(1.)  A  heam  of  light  is  received  hy  twosimilar  lenses,  (A  Ji, 
Fig.  10,)  which  iiring  tiie  light  to  a  focus  on  either  side  of 
the  interrupting tlisk  (C)  The  two  sidistanccs,  whose  sonorous 
powers  are  to  he  <romj)ared,  are  ])lac('d  in  the  recH'iving  ves.sels 
(I)  P])  (so  arrang(!d  as  to  e.vpose  e(jiial  surfaces  to  the  ac^tion 
of  the  iieam)  wiiich  communic^ate  l>y  ile.xihle  tuhes  {V  (i)  of 
etpial  length,  with  the  common  hearing-tulic  (II.)  The  re- 
ceivers (I)  E)  are  placed  upon  slides,  which  can  he  moved 
along  tlic  graduated  supports  (I  K.)  The  heams  of  light  j.ass- 
iiig  through  the  interrupting  disk  ((J)  are  alternately  cut  off  i>y 
the  swinging  of  a  pendulmn,  (L.)  Thus  a  nmsicid  tone  is 
prodjiced  alternately  from  the  suhstanee  in  D  and  from  that 


Il/,S     tlfllllllf 


i.r.i  III 

l.!M»   " 
ti.dO   " 
2.15   " 
2. in  " 
.'{.10    " 
4.01    " 
4.0»'.   " 
4.1H   " 
4.;{S   " 
4.52   '• 
4.(!!»   " 
4. 82   " 
5.(»2   " 
5.21    " 
5.24   " 
(1.50   " 


10.0(»   " 

s  tlmt  this 
t  oiU'e  ilo- 
<l('.><rnl>e(l 

'  llH.><  KillCf 

;  it  toyoii 

iOH,  (A   Ji, 

r  sido  «»f 

sonoi'on.s 

"K  vessels 

lie  sictioii 

(F  G)  ,.f 

The  re- 

e  UK t veil 

,i;h(  |iass- 

II t  off  hy 

tone  is 

oin  that 


^ 


81 

ill  K.  (>ii('nl'  tlic  rtM'(>ivt>i-s  JH  ki^pf  lit  u  ciiiiHtniit  point  iijM»n 
itfi  scale,  and  tlie  (iflicr  rwciver  in  moved  towardH  or  from  the 
focMis  of  its  Ik  •im  until  tin-  car  decides  tliat  tlu'  Konnds  pro- 
duced from  Daiid  K  arc  of  c(|ual  iiitciiHit}'.  The  relative  po- 
sitions (d"  tlie  receivers  are  then  noted. 

("2.)  Another  metluMl  of  inveHti>;ation  is  lutHcd  u|M>n  the  pro- 
diicfion  of  an  interference  td  sound,  and  the  apparatus  employed 
\n  shown  in  Fi^.  II.  The  interrupter  consists  of  a  tuiiinj^- 
fork,  (A,)  which  is  kept  in  continuous  vihration  hy  meaiiH  of 
an  eleetro-majjnet,  ( H.) 

A  powerful  heaiii  of  li^ht  is  lirought  to  a  fo«Mis  hetween  tlu' 
proiij^s  of  the  tuniii;j-fork,  (A,)  and  the  passage  (»f  the  heam  is 
more  or  less  ohstriicted  liy  the  vihration  of  tin:  opaijiie  Kcreeiis 
((/  I))  carried  \>\  the  prongs  of  the  fork. 

Ah  the  tiiinng-fork  (A)  produccH  a  Hound  hy  itH  own  vihra- 
tion, it  is  j)laced  at  a  sutlicient  distanee  away  to  he  inaudihle 
through  the  air,  and  a  system  of  lenses  is  employed  f(»r  tint  piir- 
])ose  <d'  hringing  the  undulating  heam  of  light  to  the  re<!eiving 
leiis  (E)  with  as  little  hms  as  possihle.  The  two  receivers  (F 
G)  are  attached  to  slideH  vhieli  move  upon  the  graduated  sup- 
p<»rtri  (II  I)  on  opposite  sides  of  the  axis  of  the  Ixiain,  and  the 
receivers  are  connected  hy  tlexihh'  tuhes  of  unequal  length  (K 
\j)  communicating  with  the  common  hearing-tuhe  (M.) 

The  length  of  the  tulie  (K)  is  such  thai  the  koikh'ous  vihra- 
tioiiH  from  the  receivers  (F  G)  reach  the  common  hearing-tuhe 
(M)  in  opposite  plii!><es.  Under  these  cireumHtaiices  silence  is 
produced  when  the  vibrations  in  the  receivers  (F  G)  are  of 
ecpial  intensity.  Wluiii  the  intensities  are  unequal,  a  residual 
effect  itt  perceived.  In  operating  the  instrument  the  position 
of  the  receiver  (G)  remains  (constant,  and  the  reiteiver  (F)  is 
moved  t(»  or  from  the  focus  of  the  heam  until  complete  silence 
is  pntduced.  The  relative  positions  ()f  the  two  receivers  are 
then  noted. 

(3.)  Another  mode  is  as  fidlows;  The  loudness  of  a  musical 
tone  produced  hy  the  action  of  light  is  compared  with  the 
loudness  of  a  tone  of  similar  pitch  produ(!ed  hy  electrical 
means.     A  rhettstat  introduced  into  the  circuit  enables  us  to 


"""-'' iiSn'TiSgi 


■■I 


32 

inwisiiic  til!'  iinmunt  of  rcsistiiiuro  required  to  render  the  elec- 
triciil  siMiiid  e(|iiiil  in  intensity  to  the  otlicr. 

(4.)  If  thi;  tuning-fork  (A)  in  Fijjj.  1 1  ib  thrown  into  vibra- 
tion hy  iin  unduliitory  insteud  of  an  intennittent  <Mirrent  passed 
throni^h  tlie  ekM'tro-niajjnet,  (B,)  it  is  proluihle  that  a  niusica! 
tone,  eh'ctrieallv  produceci  in  the  receiver  (F)  Ity  the  aetion  of 
tlie  same  eurrent,  wouhl  he  f(Mind  <!apahle  of  extinfinishinj;  the 
effect  produced  in  the  reeeiver  (G)  hy  tlie  action  of  the  nndu- 
latory  heani  of  li'jjht,  in  \vlii<-h  case  it  should  he  possible  to 
establish  an  acoustic  balance  between  the  effects  produced  by 
liijiit  and  electricity  l»y  introducing  sufficient  resistance  into  the 
electric  circruit.. 

f^/)o,i  //ir  Nidxre  of  tin'   Uiij^  f/tif  Proilnce  Siinoroiis  Ejfectx 
in  I}[^\'i'eiit  Siihstaih'Ci. 

In  my  paper  read  l»efore  the  American  Association  last 
August  and  in  the  present  paper  i  liiive  use  "  'le  word  "light" 
in  its  usual  rather  than  its  scientiffc  sense,  and  [  have  not  hith- 
erto attempted  to  discriminate  the  efi^ects  produced  i»y  the  dif- 
ferent (tonstitnents  of  ordinary  light,  the  tliermal,  luminous, 
and  actinic  rays.  I  find,  however,  that  the  adoption  of  the 
word  "pliotophone"  by  Mr.  Tainter  and  myself  has  led  to  the 
assumption  that  we  believe<l  the  audil)le  effects  discovered  by 
us  to  be  due  entirely  to  the  action  of  luminous  rays.  The 
meaning  we  have  uniformly  attached  to  the  words  "photo- 
pli(»ne"  and  "light"  will  be  obvious  from  the  following  pas- 
sage, (pioted   from  ni}'  Boston  paper: 

"  Although  effects  are  produced  as  aitove  shown  by  forms  of 
"  radiant  energy,  which  are  invisible,  we  have  named  the  appa- 
"  ratus  for  the  produi^tion  and  reproduction  of  sound  in  this  way 
"  the  'pliotophone'  hern  use  (in  oi'diminj  beam  of  liykt  contains 
"  tilt;  /■Ki/.s  irltirh  ni'e  oj>er(itit'ey 

To  avoid  in  future  any  inisunderstiindings  upon  this  point  we 
have  decided  to  adopt  the  term  '■'■  /'ui/ioplionr,'"  proposed  by  M. 
Mcrcadier,  as  a  general  term  siirnifying  an  apparatus  for  the 
production  of  sound  by  any  form  of  radiant  energy,  limfting 
the  words  tlierniophone,  photojyhone,  and  actinophone  to  appa- 


iider  the,  eleo- 

111  into  vil>rii- 

iirrent  pHssed 

Hint  a  iinisicjil 

file  action  of 

ifj:nisliinj,'  flip 

'•f   flio  Mtldll- 

>e   jKtssihle  to 

i  pnwlncod  l»y 

fiuice  into  the 


iiirons  KfevtK 


sociution  l^sf 
word  "hVlit" 
iHve  not  liith- 
ed  I.J  the  dif- 
ihI,  InniinoHK, 
iipfion  of  the 
has  led  to  the 
diwiovered  l»y 
s  i-Hjs.  The 
ords  "photo- 
(llowing  pas- 


1  l»y  forms  of 
led  the  appa- 
d  in  this  way 
Kjht  contains 


this  point  we 
pose*!  hy  M. 
•atUH  for  the 
•gy,  limiting 
'/'t!  to  appa- 


wA 


,    i  r 


I  li' 


*  I' 


iif^j„it, 


i  ''\ 


■■UMI 

^ 

>S 

i.^H— I 

1 

*«ii; 


3t 

ratus  for  the  production  of  sound  l»y  tluM'niiil,  Inininous,  or 
iurtini('  ruyrt  rewpectivcly. 

M.  M»'r<'u<li*'r,  in  the  course  of  his  reseureherf  in  riidiophonj, 
purised  an  intermittent  beam  from  an  electric  lamp  through  a 
()rism,  and  then  e\aniine<l  the  audihle  effectw  produced  in  dif- 
ferent parts  of  the  spectrum.    (( 'onijtfen  /^em/tm,  Dee.  6th,  1880.) 

We  have  repeate<l  this  experiment,  usinj^  the  sun  as  (»ur  source 
of  radiation,  and  have  obtained  results  somewhat  different  from 
those  noted  l»y  M.  Mercadier. 

(1.)  A  l>eam  of  sunlight  was  retlec^ted  from  a  heliostat  (A, 
Fig.  12)  through  an  achromatic  lens,  (B,)  so  as  t<»  form  an 
image  of  the  sun  upon  the  slit  (C.) 

Th(!  Iteam  then  })assed  through  another  achromatic  lens  (D) 
and  through  a  hisidphide  of  carbon  ])rism,  (E,)  forming  a  spec- 
trum of  great  intensity,  which,  when  focused  upon  a  screen, 
was  found  to  l»e  sutHiMently  pure  to  show  the  principal  absorp- 
tion lines  of  the  solar  spectrum. 

Tile  disk-interrupter  (F)  was  then  turned  with  sutticient  ra- 
pidity to  produce  from  live  to  six  hundred  interruptions  of  the 
light  per  second,  and  the  spe(;trum  was  explored  with  the  re- 
ceiver, (G,)  which  was  so  arranged  that  the  lamp-black  surface 
expt)sed  was  limited  by  a  slit,  as  shown. 

Under  these  (arcumstances  sounds  were  olttained  in  every 
part  of  the  visible  spectrum,  (excrepting  the  extreme  half  of  the 
violet,)  as  well  iw  in  the  ultrn-red.  A  continuous  increase  in 
the  htudness  of  the  soimd  was  observed  up<»n  nutving  the  re- 
ceiver (G)  grr,dually  from  the  violet  into  the  ultra-red.  The 
point  of  maximnin  sound  lay  very  far  out  in  the  ultra-red.  Be- 
yond this  point  the  sound  Itegan  to  decrease,  and  then  stopped 
so  su(!;lenly  that  a  very  slight  motion  of  the  receiver  (G)  made 
all  the  difference  l»etween  almost  nuiximum  sound  and  (complete 
silence.* 

('2.)  The  lamp-blacked  wire  gauze  was  then  removed  and  the 
interior  of  the  receiver  (G)  was  filled  with  red  worsted.  Upon 
exploring  the  spcurtrnui  as  l»efore,  entirely  different  results  were 
obtained.     The  nuixinuun  effect  was  produced  in  the  green  at 

*  The  resultH  obtained  in  this  uud  subsequent  experiments  are  shown  in  a 
tabulated  form  iu  Fig.  14. 


38 


m\ 


that  j>!irt  wluTc  the  red  worstt'd  iipju'ured  to  I>t'  lilack.  On 
cither  side  of  this  pniiit  the  seuiid  <;raduidly  died  away,  heeoiii- 
iii^f  iiuuidihh^  on  tlie  one  side  in  the  middle  of  the  indigo,  and 
on  the  other  at  a  short  distance  outside  the  edj^e  of  the  red. 

(;i.)  Upon  8nl»stitnting  fjjrcen  silk  for  red  worsted  the  limits 
of  andition  appeared  to  he  the  middle  of  the  Itlue  and  a  point 
a  short  distancre  ont  in  \ho  ultra-red.     Maxinnmi  in  the  r<'d. 

(4.)  Some  hard-ruhh(^r  shavin<ifs  were  now  placed  in  the  re- 
ceiver (G.)  Th(!  limits  of  iiudihilitv  appeared  t(»  be  on  the  one 
han<l  the  junction  of  the  fjjreen  and  hlue,  and  <tn  the  other  the 
outside  edf^e(»f  the  nn].  Maxinntm  in  the  yellow.  Mr.  Tainter 
thou<i;ht  he  couhl  hear  a  little  way  into  the  ultra-red,  and  to 
his  ear  the  inuximum  was  alxtut  the  junction  of  the  red  and 
orange.* 

(5.)  A  test-tuhe  containing  the  vapor  of  sulphiu-ic  ether  was 
then  substituted  for  the  recreiver  (G.)  Conmiencing  at  the 
violet  end,  the  test  tube  was  gradually  moved  down  the  spec- 
trum and  out  into  the  ultra-red  without  audible  effec-t,  but 
when  a  certain  point  far  out  in  the  \dtra-red  was  reached  a  dis- 
tinct musical  tone  suddenly  made  its  appearantre,  whicth  disap- 
peared as  suddenly  on  moving  the  test-tube  a  very  little 
further  on. 

(6.)  Upon  exploring  the  spectrum  with  a  test-tube  contain- 
ing the  vapor  of  iodine  the  limits  of  audibility  appeared  to  be 
the  middle  of  the  red  and  the  junction  of  the  blue  and  indigo, 
Maxinnnn  in  the  green. 

(7.)  A  test-tube  containing  peroxide  of  nitrogen  was  substi- 
tuted for  that  containing  iodine.  Distinct  sounds  were  ob- 
tained in  all  part^  of  the  visible  spectrum,  but  no  sounds  were 
observed  in  the  ultra- red. 

The  sounds  were  well  marked  in  all  parts  of  the  violet,  and 
I  even  fancied  that  the  audible  effect  extended  a  little  way  into 
the  nltra-violet,  but  of  this  I  cannot  be  certain.  Upon  exam- 
ining the  absorption  spectrum  of  peroxide  of  nitr<»gen  it  was  at 
once  observed  that  the  maximum  sound  was  produced  in  that 
part  of  the  spectrum  where  the  greatest  nund)er  of  absorption 
lines  made  their  appearance. 

*  In  the  diagram  Fig.  14  the  mean  of  these  readiugs  is  shown. 


luck.  Oil 
iiy,  1  »»•<'(  nii- 
i<li<r<>,  iuid 
lie  red. 
tiic  limits 
lid  11  point 
tlio  red. 
in  the  tr- 
im the  one 
other  the 
[r.  Tainter 
;d,  and  to 
e  red  and 

ether  was 
ig  at  the 
tiie  spec- 
iffect,  hut 
ihed  a  dis- 
ich  disap- 
ery   little 

e  contain- 
red  to  he 
id  indigo. 

as  suhsti- 

were  oh- 

iiids  were 

iolet,  and 
!  way  into 
on  exam- 
it  wa.s  at 
3d  in  that 
hsorption 

wn. 


i; 


41 

(8.)  The  fipoctriim  wiis  now  oxplorod  l»y  a  solftiium  (;olI,  unti 
the  utxlihlc  effects  were  ohserved  hy  ineiiiiH  of  ti  teh^plioiie  in 
tlie  siinie  •rtilvaiiic  (rircuit  with  the  cell.  The  niuxinnnn  effect 
was  })r«Mhi<*etl  in  the  re<l  alKtut  its  junction  with  the  orunjie. 
The  aiidiitle  effect  exten<le<l  a  litth>  way  into  the  ultra  red  on 
the  one  hand  and  up  as  hi^h  as  the  middle  of  the  \  iolet  on  the 
other. 

Although  the  experiments  so  far  made  can  only  he  consi<lered 
as  preliminary  t<»  (tthers  of  a  more  retinwl  nature,  I  think  we 
are  warranted  in  concludiu}^  that  the  ixttiire  of  the  rtiyn  that 
produce  sonorouH  effects  in  (Hjfiiviit  HnhstiiiireK  depenih  ujnm 
th^  nature  oj'  the  auhtttiitireti  that  ure  e,vjh>ftetl  to  the  tieiiin,  (in<l 
that  tfie  Hutiutln  ure  in  every  rune  due  to  those  ruytt  of  the  t<j}ee- 
truni  that  are  ahsorU'd  l>y  the  body. 


The  Hpectrophone. 

Our  experiments  upon  the  ranj^je  of  audibility  of  different 
Buhstanees  in  the  spectrum  have  led  us  to  the  construction  of  a 
new  instrument  for  use  in  spectrum  analysis,  whicth  was  de- 
scribed and  exhibited  to  the  Philosophit;al  SiMiiety  t»f  Washing- 
ton last  Saturday.*  The  eye-piece  of  a  spectrosc^ope  is  re- 
moved, and  sensitive  substances  are  pla<tetl  in  the  focal  point  of 
the  instrument  Itehind  an  opaque  diaphragm  containing  a  slit. 
These  substances  are  put  in  connnunication  with  the  ear  by 
means  of  a  hearing-tube,  and  thus  (he  instrument  is  converte<l 
into  a  veritable  "  spectrophone,"  lilce  that  shown  in  Fig.  13. 

Suppose  we  smoke  the  interior  of  our  spectrophonie  receiver, 
and  fill  the  cavity  with  peroxide  (d  nitrogen  gas.  We  have 
then  a  combination  that  gives  us  good  sounds  in  all  parts  of 
the  spectrum,  (visible  and  invisible,)  except  the  ultra  violet. 
Now,  pass  a  rapidly-interrupted  beam  of  light  through  some 
substance  whose  ab8i>rption  spectrum  is  to  be  investigated,  and 
bands  of  sound  and  silence  are  observed  upon  exploring  the 
spectrum,  the  silent  posititMK-'  corresponding  to  the  absorption 
bands.  Of  course,  the  ear  camntt  for  one  moment  compete 
with  the  eye  in  the  examination  of  the  visilde  part  of  the  spec- 

♦Proc.  of  Phil.  8oc.  of  Washingtou,  April  10,  1881. 


42 


J  1 


tnun  ;  1»ut  in  tin-  iiiviHiMc  part  Im'voiuI  the  red,  wlicri'  tlu'  cyo 
iK  iiHclcKH,  tlu'  I'jir  is  iiivulii;ii>lc.  fii  working  in  tliis  rv^um  itf 
llif  spfctrnni,  liiinp-MiKk  ulonc  iniiv  lie  iist'd  in  tlic  ripcctnt- 
piinnic  mu'ivor.  Indcj-cl,  tlic  Honnd.-t  pi-i.dii«r(>d  l.y  tliis  sidi- 
Htanco  in  tlio  nltni-rcd  nw.  w»  well  nmrkod  m  to  ••i.nHtitnt(^  (nir 
inHtnnncnt  .-i  most  rclialdc  and  convtMiicnf  sniisfitiit*-  fur  tlu* 
tlH-iino-pilc.  A  few  ♦•xpcrinit-ntrt  tliat  have  \n>i'u  made  may  he 
inten'sting. 

(1.)  The  infernipte<l  heaui  was  filtered  through  a  saturated 
solution  of  alum. 

Result :  Tlu^  ranjjfe  ..f  audihility  in  the  idtra-red  was  slif,ditly 
redueed  hj  the  almorption  of  a  narrow  hand  of  the  rays  of  lowest 
refraiif^ihility.  The  sounds  in  the  visihle  part  of  the  spectrum 
seemed  to  he  unaffected. 

(2.)  A  thin  sheet  of  hard  ruhher  was  interposed  in  the  path 
of  the  heam. 

Result :  Well-marked  sounds  in  every  part  of  tlm  ultra-red. 
No  sounds  in  the  visihle  part  of  (he  spectrum,  exct^pting  the 
t^vt^eme  half  of  the  red. 

These  experiments  reveal  the  cause  of  the  curious  fact  al- 
luded to  in  my  paper  read  l.efore  the  American  Association 
last  August — that  sounds  were  heard  from  selenium  when  the 
l«»am  was  filtered  through  hoth  hard  ruhher  and  alum  at  the 
same  time.     (See  table  of  results  in  Fig.  14.) 

(3.)  A  solution  of  ainmonia-sulphatc  of  copper  was  tried. 
Result :  When  placed  in  the  path  of  the  heam  the  spectrum 
disappeared,  with  the  exception  of  the  blue  and  violet  end.  To 
the  eye  the  spectrum  was  thus  reduced  to  a  single  broad  band 
of  blue-violet  light.  To  the  ear,  however,  the  spectrum  re- 
vealed itself  as  two  bands  of  sound  with  a  broad  space  of  silence 
between.  The  invisible  rays  transnutted  constituted  a  narrow 
band  just  outside  the  red. 

I  think  I  have  said  enough  to  (ionvince  you  of  the  value  of 
this  new  method  of  examination,  but  I  do  not  wish  you  to 
understand  that  we  look  upon  our  i-esults  as  Ity  any  means 
complete.  It  is  often  more  interesting  to  observe  the  first  tot- 
terings  of  a  child  than  to  watch  the  firm  tj-ead  of  a  full- 
grown  man,  and  I  feel  that  our  first  footsteps  in  this  new  field 


,^% 


i:i 


*i'o  tlio  eye 
region  of 
u'  rtpcctro- 
'  this  8ul»- 
rttifntc^  our 
tf  for  thd 
kI(i  iiiiiy  lie 

I  Hutiiruted 
■)iH  Hlif^litly 

H  of  loWCHt 

?  wpcrtniiu 

II  tl»'  path 

!  iiltni  red. 
'ptiiig  the 

18  fact  nl- 

LHKociution 

when  the 

lint  at  tiie 

3  tried, 
spectrnm 
tend.  To 
'oad  hand 
otrum  re- 
of  silence 
a  narrow 

value  of 
sh  you  to 
ny  means 
5  first  tot- 
.f  a  full- 
new  field 


i-i 


1 


46 

of  w'.irnrc  may  have  more  of  iiitrrcHt  to  ynii  tlimr  tlic  fiilliT 
i-i'kiiUh  of  matiin'  rcHciircli.  TIiIk  iiniHt  l»(^  my  cxciiw  for  liavin^ 
(Iwi'lt  so  loiij;'  upon  the  «lctiiilH  of  iiicoin|>lft(>  »'X|ifriiii('iitH. 

I  n'roj^iiizc  till"  fiict  tliiit  the  HjHM'troplioiic  iiniHt  over  n-miiiii 
H  inert'  atljiinrt  to  the  HjH'ctroHeojH',  l»iit  I  aiiti(-i|>Ht(>  that  it  h»K 
II  wiile  mill  in)h-]HMi(lent  lieM  of  nKefiihieHs  in  thu  iiivehti^^iitioii 
of  iilisorptioii  Hpectni  in  the  iiltni-red. 


*i' 


^jj^jStiiiiMiMSmiiiltmiTiU  iiMniiail'i  lii'"'  '"■'"''  1'ii'ini.u.i.Mll 


